Film



- Patented Mar. 1 7, 1942 Albert Hershberger, Buffalo, N. Y., assignor to E. I. du Pont de Nemours & CompanyI Wilmington, DeL, a corporation of Delaware 'No Drawing. Application February 2, 1940,

I SerialNo. 316,937

7 Claims.

This invention relates to the stripping of organic solvent soluble dry cast polyvinyl alcohol substitution derivative films, especially continuous webs, from the casting surface. More particularly it appertains to the incorporation of materials, for convenience called stripping agents, in polyvinyl acetal dopes for facilitating separation of the cast film from the casting wheel.

Polyvinyl acetals which have been applied in solution to the surface of such materials as wood, metals, ceramics, plastics and the like, and the solvent evaporated, possess considerable adhesion therefor. If applied for protective purposes this inherent adhesion (between the polyvinyl acetal and the base object upon which it is coated) would be desirable, but such an adhesion is very undesirable when the base is employed only as a temporary support during the solvent removal,

as in the casting of self-supporting film like sheet wrapping and similar material.

In general dry casting procedures involve the laying of a film of dope (solution of the cellulose derivative) on to a casting wheel mounted for rotation and arranged for the removal of the solvent from the dope during its rotation. The means for the solvent removal may include heating the surface of the wheel or surfaces adjacent thereto, together withdevices such as fans for carrying the solvent or solvent-laden atmosphere away from the film. Ordinarily the speed of the wheel and the degree of heating are adjusted so that the desired amount of solvent is removed in somewhat less than one revolution of the casting wheel. Nickel, chromium, stainless steel and other metals and alloys are generally used for surfaces of the casting wheels.

In. attempting to carry out a continuous casting operation of this type with polyvinl acetal dopes, it was found that the adhesion between the partially dried polyvinyl acetal film and the casting surface was a frequent source of trouble. It caused the film to break or tear at the place where the film was being stripped from the wheel. This action was especially troublesome at the edges of the film. Even when the film did not tear or break, distortion and deformation were quite common due to the high tension required for pulling the film away from the casting surface.

A type of material has now been discovered which greatly decreases the adhesion between such partially dried film and the metal casting surface. With the preferred members of the group the adhesic: is reduced to such an extent that only a very slight tension is required to strip the film from the casting surface.

This invention had for an object the manufacture of transparent, strongand flexible polyvinyl acetal sheets or films which were undeformed and/or undistorted and/or unmarred, and the production of compositions of matter suitable for the casting thereof. A further object was to dry cast benzene methanol (60:40) and methanol soluble polyvinyl acetals into thin, strong, smooth, flexible sheets capable of being easily stripped from the casting wheel while retaining 5% to 15% of the solvent of the casting dope. Further objects were to devise a means of decreasing the adhesion between a dry cast, organic solvent soluble, polyvinyl alcohol substitution derivative and the casting surface, to facilitate the removal of partially dried, dry cast,

polyvinyl alcohol substitution derivatives from casting wheels and belts, and to find materials which could be added to polyvinyl acetal dry casting solutions to make removal of the film easy when only a small proportion of the solvent remains. A general advance in the art, and other objects which will appear hereinafter, are also contemplated.

It has now been discovered that the salts,

particularly the alkyl (including hydroxyalkyl and aralkyl) amine and inorganic salts of fatty acids containing more than 6 carbon atoms, may

be incorporated in casting dopes with the re- Example I To a solution containing 20% polyvinyl acetal (prepared from polyvinyl alcohol and formaldehyde) dissolved in a benzene-methanol 60:40 solvent mixture, was added 3% (based on the polyvinyl acetal present) sodium oleate. Film was cast from this solution onto a nickel surface and dried thereon. It possessed only a slight adhesion to the nickel surface, as compared to film cast from a similar polyvinyl acetal solution to which no sodium oleate had been added.

Example II A "film was prepared from ,a dope consisting of 19% of polyvinylbenzal-butyral (hydroxyl 15%, benzal 25%, isobutyral 55%, acetyl 5%), 1% of castor oil, and 80% of methanol.' Into this dope there was uniformly incorporated 2.5% (based on the solids content of the dope) calcium oleate. The resulting material was cast on a stainless steel casting wheel, and all but 5% of the non-solids (the methanol) was evaporated, and the resulting film stripped from the casting wheel. It stripped very easily compared with a similar film containing no stripping agent. The film obtained was highly transparent and durable. Similar results were obtained with the sodium, potassium, calcium, magnesium, and'like metal salts of lauric acid, myristic acid and palmitic acid.

Example 111 A dope was prepared consisting of 15.8% of polyvinyl butyral (hydroxyl 19%, butyral 80%, acetyl 1%) 4.2% castor oil, and 80% of methanol. To this dope there was added 3% (based on the non-volatile materials in the dope) of triethanolamine stearate. A thin layer of this I dope was cast on a nickel casting wheel and the volatile solvent evaporated. A glass-clear transparent film .001 inch thick, and especially suitable for wrapping woven cotton goods, resulted. It was easily stripped from the casting surface. Similar results were obtained with other alkylamine acid salts, such as the triethanolamine laurates, myristates, palmitates and oleates. The corresponding monoethanolamine and diethanolamine salts were also very satisfactory.

.Ezcample IV A dope consisting of 17% of polyvinyl isobutyral (hydroxyl 14%, isobutyral 85%, acetyl 1%). 3% castor oil, and 80% of methanol, was cast in a continuous manner upon the polished surface of a large diameter chromium-surfaced drum, Before casting 3% (based on the filmforming material and plasticizer content) of benzyl-trimethyl ammonium myristate was incorporated in the dope. Upon evaporation of the solution, a highly transparent and durable film was obtained which was easily (compared with the same composition when no stripping agent was present) stripped from the casting drum. Similar results were obtained with other alkylamine fatty acid salts such as benzyl-trimethyl ammonium laurate, palmitate, oleate and stearate.

Example V lilxample VI To a solution containing 20% polyvinyl acetal (prepared from polyvinyl alcohol and butyraldehyde) dissolved in methanol, was added 3% barium myristate (on the basis of the polyvinyl acetal). Filmcast from this solution onto a nickel surface and dried possessed only a slight adhesion to the nickel surface; when compared to film cast from a similar polyvinyl acetal solution to whichno barium myristate had been added. Barium oleate was also found to be satisfactory as a stripping agent.

Example VII A dope consisting of:

. Parts Polyvinyl butyral 6.3 Polyvinyl isobutyral 6.8 Polyvinyl formal 3.8 Castor oil (crystal grade) 3. 1 Methanol 70.0 Ethyl formate 10.0

in which had been incorporated 3.2% of zinc oleate (based on the total film-forming and plasticizing ingredient material), was cast continuously on the polished surface of a rotating nickel cylinder ten feet in diameter. The volatile materials were evaporated, and the resulting film stripped from the surface of the casting wheel. The film, which had a thickness of 0.0013 inch, was very clear, could be satisfactorily sewed, had excellent tear resistance, and was easily removed from the surface of the casting wheel. Zinc myristate and laurate were also very satisfactory.

Example VIII A film was prepared by evaporating the solvent from a composition consisting of 17% polyvinyl formal (hydroxyl 2%, formal 90%, acetyl 8%), 3% castor oil, 64% methyl acetate, 2% water and 14% methanol, to which had been added potassium oleate. The film, which stripped easily from the polished metal casting surface, was found to be extremely durable and not to tear readily under a wide variety of temperatures.

Example IX A dope consisting of:

Per cent Polyvinyl butyral (Example III type) 10 Polyvinyl formal (Example V type) 5 Castor oil 3 Methanol 33 Benzene 49 *Thut is, hydroxyl corresponding to 19% polyvinyl alcohol. butyral corresponding to 80% polyvinyl butyral and acetal corresponding to 1% polyvinyl acetate.

in which had been incorporated 2% (based on the polyvinyl acetal) lithium stearate, was cast continuously on the polished surface of a rotating nickel cylinder six feet in diameter. The volatile materials were evaporated, and the resulting film stripped from the surface of the casting wheel. The film, which had a thickness of .00088 inch. was very clear, could be satisfactorily sewed, and had excellent tear resistance. It was undeformed, unmarred, and undistorted. It was stripped from the casting wheel very easily. Similar results were obtained with the sodium, potassium, calcium, zinc, and like salts of stearic acid.

The dope or polyvinyl acetal solution contain- .ing the strip agents of the invention may be handled and cast in the same manner as the organic solvent dry casting dopes previously known to the art.

The most marked decrease in adhesion between the film and the casting wheel surface has been noted when the casting surface was nickel.

In the preferred mode of operation the film is stripped from the casting wheel while it still contains to of the solvent.

This invention is not limited to the polyvinyl acetals of the specific examples, although it peculiarly appertains thereto. In general it is, with considerable efficacy, applicable to all kinds of polyvinyl acetals which are soluble in organic liquids (solvents).

Although many plasticizers for polyvinyl ace- I talshave been suggested heretofore, castor oil is preferred for thin films.

For convenience of consideration, the materials of this invention which decrease the adhesion between a dry cast polyvinyl acetal film and the surface upon which the film is cast and dried (or partially dried), may be divided into two subgeneric groups, namely, alkyl amine salts of fatty acids containing more than six carbon atoms, and inorganic salts of fatty acids containing more than six carbon atoms.

The alkyl amine salts of fatty acids comprehended by the present invention are those of fatty acids containing at least seven carbon atoms. Preferably the fatty acids contain 12 to carbon atoms or more. Especially suitable for the purposes of the present invention are the benzyl trimethyl ammonium salts of lauric, myristic, palmitic, oleic and stearic acids. The results obtained with the triethanolamine, diethanolamine and monoethanolamine salts of the aforementioned acids are outstanding.

The inorganic salts of fatty acids suitable for use in the operation of this invention are the sodium, potassium, calcium, magnesium and like metal salts of fatty acids containing at least seven carbon atoms. The salts formed by metals from Groups I and II of the Periodic Table are especially desirable. Preferably the-fatty acids contain from 12 to 20 carbon atoms or more. Especially suitable for the purposes of the present invention are the sodium and calcium salts of lauric, myristic, palmitic and oleic acids. In certain cases the calcium salts of the fatty acids are preferred when reduction of water (or moisture) sensitivity in the polyvinyl acetal film is desired.

In the preferred embodiments of the invention the stripping agent is present in an amount within the range of 0.5% to 5% by weight on the basis of the polyvinyl acetal. Higher concentrations are not excluded, and may be used in order to secure specific results.

The strip agents may be used singly or in any combination, and it is to be understood that the invention is not limited to the specific materials listed above or in the specific examples.

. The stripping agents of this invention may be incorporated in the casting dope in any desired manner. If preferred, they may be predissolved and the solvent used for dissolving the polyvinyl acetal. Ordinarily they are incorporated into the solvent simultaneously with the polyvinyl acetal. In some instances it has been found convenient to add the-stripping agent to the solution of the polyvinyl acetal.

Many of the advantages of the present invention will be apparent from the preceding portion of the specification. Specific mention ay be made of the fact that by this invention it is possible to easily, speedily and satisfactorily remove the partially dried, dry cast polyvinyl acetal film from a metal or alloy casting wheel at elevated temperatures, and that all danger of tearing or deforming the dry cast film at the stripping point (or line) due to the pull required to separate the film from the casting surface has been removed.

The advantage of incorporating in a casting solution a material which decreases the adhesion between the polyvinyl acetal film cast therefrom and the casting surface without undesirably affecting the transparency and surface characteristics of the film, will be obvious to those skilled in the art.

As many apparently widely different embodiments of this invention may be made without departing from the spirit and scope thereof, it is to be understood that this invention is not limited to the specific embodiments thereof except as defined in the appended claims;

I claim:

1. An organic solvent dry casting solution of polyvinyl acetal containing 0.5% to 5% (based on the solids contentof the casting solution) of a1- kylamine salt of a fatty acid containing 12 to 20 atoms. V

2. The product of claim 1 when the polyvinyl acetal is polyvinyl butyral and the fatty acid salt is triethanolamine stearate.

3. The product of claim 1 when the polyvinyl acetal is polyvinyl butyral and the fatty acid salt is benzyl-trimethyl ammonium myristate.

4. A film of polyvinyl acetal containing 0.5% to 5% of triethanolamine stearate and 5% to 15% casting solvent.

5. A film of polyvinyl acetal containing 0.5% to 5% of benzyl-trimethyl ammonium myristate and 5% to 15% casting solvent.

6. An organic solvent dry casting solution of polyvinyl butyralcontaining 0.5% to 5% (based on the solids content of the casting solution) of alkyl amine salt of fatty acid, the fatty acid radical of which contains 12 to 20 carbon atoms.

7. Anorganic solvent dry casting solution of polyvinyl acetal containing 0.5% to 5% (based on the solids content of the casting solution) of 'alkyl amine salt of fatty acid, the fatty acid radical of which contains more than 6 carbon atoms.

ALBERT HERSHBERGER. 

